Unexpected Kinship in Plants' Family Tree

In the last few years, botanists have managed to ink in some key branches of the plant kingdom's family tree. They've traced the evolution of flowers in recent years, for instance, but the history of more ancient plants has remained enigmatic. Now researchers think they have identified some of the older branches of the family tree. They report that, unexpectedly, seed-bearing plants share a common ancestor with both horsetails and ferns. This close kinship may help clarify how plants evolved.

The study focused on vascular plants, those with special tissues for transporting food and water. To trace their genealogy, a team led by botanist Kathleen Pryer of the Field Museum of Natural History in Chicago examined 35 plants that represented all the main branches of living land plants. Pryer and her colleagues assembled and analyzed a data set of 136 anatomical traits, as well as DNA sequences of four genes, three from the chloroplast and one from the nucleus. Horsetails are more closely related to ferns than anything else, the researchers report in the 1 February issue of Nature. "Where horsetails fit in the picture was a big mystery," Pryer says, "because they are so different in form from everything else."

Another conclusion was that horsetails together with ferns form a major group of plants that are the closest living relatives to seed plants. This refutes a general idea of plant evolution that many botanists have, Pryer says. Most botanists think of ferns and horsetails as intermediate stages in plant evolution that eventually led to seed plants. As a result, many aspects of seed plants are commonly regarded as having been derived from a fern ancestor. But that clearly could not have happened, Pryer says, if the ferns and horsetails diverged as a independent group that has had a long evolutionary history separate from seed plants.

By finding that ferns and horsetails are more closely related to each other than to seed plants, Pryer and colleagues have "added a new branch to the tree," says Brent Mishler of the University of California, Berkeley. And Paul Kenrick, a paleobotanist at the Natural History Museum in London, notes that a close relationship between ferns and horsetails could help decipher the evolution of such basic features of plant anatomy as leaves and branching.